Spectrometric instruments are used for a variety of applications usually associated with analyses of materials. A spectrum is generated in interaction with a sample material to effect a spectral beam that is characteristic of the sample and impinged on a photodetector. Modern instruments include a computer that is receptive of spectral data from the detector to generate and compare spectral information associated with the materials. The spectrum may be generated, for example, by a dispersion element such as a prism or a holographic grating that spectrally disperses light passed by a sample or received from a plasma or other excitation source containing sample material. Another type of instrument incorporates a time varying optical interference system, in which an interference pattern of light is produced and passed through a sample material that modifies the pattern. In such an instrument Fourier transform computations are applied to the detector signals to transform the modified light pattern into spectral data. The Fourier transform instrument is most commonly operated in the infrared range, in which case it is known as an "FTIR" instrument.
With improvements in optics, detectors and computerization, there has evolved an ability to perform very precise measurements. Examples are an absorption spectrophotometer, a polychromator or an FTIR instrument that use chemometric mathematical analysis to measure octane number in gasolines. Differences in octane number are associated with subtle differences in near infrared (IR) absorption spectra. The very small changes in spectral characteristics cannot effectively be detected directly by personnel, and computerized automation is a necessity. It also is desirable for such spectral measurements to be effected continuously on line. Thus there is an interest in utilizing advanced spectrometry methods for analytical chemistry.
A problem with high precision measurements is that instruments vary from each other, and each instrument varies or drifts with time. One aspect of the problem is achieving and maintaining wavelength calibration. A more subtle aspect is that the instruments have intrinsic characteristics that are associated with spectral profiles and are individual to each instrument and may vary with time. Intrinsic characteristics of the instrument distort the data, rendering comparisons inaccurate. In an instrument such as a polychromator with a dispersion grating, an intrinsic characteristic is typified by the profile of spectral data representing a very narrow, sharp spectral line. Such a profile has an intrinsic shape and line width wider than the actual line, due to the fundamental optical design as well as diffraction effects and other imperfections in the optics and (to a lesser extent) electronics in the instrument. An actual intrinsic profile may not be symmetrical. In a grating polychromator and similar instruments, the instrument profile from a narrow line source is often similar to a Gaussian profile. For other instruments such as FTIR, the intrinsic profile attributable to aperture size at the limit of resolution is more rectangular.
U.S. Pat. No. 5,303,165 (Ganz et al) of the present assignee discloses a method and apparatus for standardizing a spectrometric instrument having a characteristic intrinsic profile of spectral line shape for a hypothetically thin spectral line in a selected spectral range. The instrument includes a line source of at least one narrow spectral line that has an associated line width substantially narrower than the width of the intrinsic profile. A target profile is specified having a spectral line shape for a hypothetically sharp spectral line, for example a Gaussian profile of width similar to that of the intrinsic width. The instrument is operated initially with the line source to produce a set of profile data for the line such that the data is representative of the intrinsic profile. A transformation filter is computed for transforming the profile data to a corresponding target profile, and is saved. The instrument then is operated normally with a sample source to produce sample data representative of a sample spectrum. The transformation filter is applied to the sample data to generate standardized data representative of the sample. Such standardized data is substantially the same as that obtained from the same sample material with any similar instrument, and repeatedly with the same instrument over time.
Standardization according to the foregoing patent is utilized particularly with an instrument having the capability to utilize a source of one or more spectral lines, such as a Fabry-Perot etalon placed in the beam from the light source in place of a sample, so as to pass the spectral line to the grating or other dispersion element. In the case of certain other instruments including FTIR, it is possible but cumbersome to utilize such a line source for such a standardization technique.
Conventional FTIR instruments are taught in textbooks such as "Fourier Transform Infrared Spectrometry" by P. R. Griffiths and J. A. de Haseth (Wiley, 1986). In these instruments, an interference pattern of light is produced with a Michaelson or similar interferometer comprising a beam splitter which is a partial reflector that splits white light into two beams. These beams are reflected back and recombined at the beam splitter. The path length of one of the beams is varied with time to produce a time-varied interference pattern. This light pattern is directed through an angle-selecting aperture and thence through a sample material that modifies the pattern. Fourier transform computations transform the modified pattern into spectral data representing intensity vs. wavenumber. (Wavenumber is reciprocal of wavelength and proportional to frequency.) The aperture generally should be as small as practical to minimize distortion of the spectral beam due to finite size of the aperture and the size and configuration of the light source, and other instrument features. The distortion has several aspects: ordinary broadening which is predictable but not generally corrected; wavelength shift; and pattern shape change due to reflections, alignment, flatness of mirrors, light source geometry, and the like. Distortions related to wavelength shift and shape change are addressed by the present invention. A very small aperture may sufficiently minimize distortion, but passes the light at too low an intensity, thereby requiring long term operations for sufficient spectral data. Therefore, normal operations are made with a larger aperture that introduces more distortion.
A further characteristic of FTIR is that the limit of resolution (minimum line width) attributable to the aperture is a function of the spectral wavenumber, in particular being proportional to the wavenumber, viz. greater line width at higher wavenumber. To apply the transformation of the aforementioned U.S. Pat. No. 5,303,165 would require defining and storing a separate target profile for many increments in the wavenumber scale in the selected spectral range, and operating the instrument repeatedly or with a source of many lines to obtain the corresponding intrinsic profiles that would be applied individually to test data. This could be cumbersome for frequent restandardizations, and may substantially lengthen the computation times for every analysis with the instrument.
An object of the invention is to provide a spectrometric instrument with a novel means for effecting standardized spectral information. Another object is to provide a novel method for standardizing spectral information from spectrometric instruments that intrinsically distort the data. Other objects are to provide a novel method and a novel means for transforming spectral data of the instrument so that spectral information is idealized for comparison with that of the same instrument at other times, or with other similar instruments. A further object is to provide such standardizing for an instrument where distortion of data is dependent on spectral wavenumber. Yet another object is to provide a computer readable storage medium with means for effecting standardized spectral information in instruments that incorporate computers. A particular object is to provide such standardizing for an interferometer instrument that incorporates Fourier transform.